Catalyst systems for the polymerization of ethylene or mixtures thereof with higher alpha-olefins are known and described in several patents, and the polymers obtained show a rather narrow distribution of molecular weights. Said polymers are suitable for injection molding and for other applications but are not satisfactory for the manufacture of articles by extrusion or blowing. In fact, it is known that, due to the narrow distribution of the molecular weights, the polymers are subject to breaking phenomena of the molten mass during the forming process and, furthermore, the articles prepared according to the above-mentioned techniques present knurlings.
In practice, the value of the ratio MI N/MI E is assumed as a measure of the breadth of the molecular weight distribution, MI N and MI E being the melt indexes of the polymer measured at 190.degree. C. with a weight of 10.00 Kg and of 2.16 Kg respectively (ASTM D 1238). Polymers with about the same value of melt index MI E have broader molecular weight distribution if the value of the MI N is higher.
Different methods can be used in order to obtain polymers with a rather broad molecular weight distribution. The simplest consists in suitably modifying the catalyst system, but such method often involves several drawbacks, such as, for example, an excessive decrease of the catalyst activity and/or difficulties in the regulation of the molecular weight of polymers.